Our `*.page.{server|route}.js` files should not have exports that are unknown to `vite-plugin-ssr`.

For example:

```js
// *.page.{server|route}.js

// `someStuff` is an unknown export; `vite-plugin-ssr` will show a warning.
export { someStuff }

function someStuff() {
 // ...
}
```

Instead, we should define & export `someStuff` outside of `*.page.{server|route}.js`:

```diff
  // *.page.{server|route}.js

+ import { someStuff } from './someStuff'

- export { someStuff }
-
- function someStuff() {
-  // ...
- }
```
```diff
+ // someStuff.js
+
+ export { someStuff }
+
+ function someStuff() {
+  // ...
+ }
```

Note that `.page.js` files are allowed to have custom exports (because it enables features such as page-specific static meta data, e.g. [Markdown - `<head>`](https://vite-plugin-ssr.com/markdown#head)).

> **Why?**
> `vite-plugin-ssr` forbids unknown exports in order to improve your app's code readability:
> if someone unfamiliar with your code reads `export { someStuff }`,
> then he'll be unsure whether `someStuff` has an effect on `vite-plugin-ssr` or not.
> Forbidding custom exports makes your app's code clearer and thus easier to understand.
